SMTPS is not a proprietary protocol and not an extension of SMTP. It is a way to secure SMTP at the transport layer. SMTPS uses port 465.

This means that the client and server speak normal SMTP at the application layer, but the connection is secured by SSL or TLS. This happens when the connection is established before any mail data has been exchanged. Since whether or not to use SSL or TLS is not negotiated by the peers, SMTPS services are usually reachable on a dedicated port of their own.

Originally, in early 1997, the Internet Assigned Numbers Authority registered 465 for SMTPS.[1]
By the end of 1998, this was revoked when STARTTLS had been specified.[2] With STARTTLS, the same port can be used with or without TLS. SMTP was seen as particularly important, because clients of this protocol are often other mail servers, which can not know whether a server they wish to communicate with will have a separate port for TLS.[3] The port 465 is registered for both Source-Specific Multicast audio and video[4][5] and submissions (aka SMTPS) as of RFC 8314.[6]

In 2014, many services continue to offer the deprecated SMTPS interface on port 465 in addition to (or instead of) the message submission interface on the port 587 defined by RFC 6409.[7] Service providers that maintain port 465 do so because[8] older Microsoft applications (including Entourage v10.0 and its successor, Outlook for Mac 2011) do not support STARTTLS,[9] and thus do not support the SMTP submission standard (ESMTPS on port 587). The only way for service providers to offer those clients an encrypted connection is to maintain port 465.

RFC 8314 "Cleartext Considered Obsolete: Use of TLS for Email Submission and Access"[10] reinstates the registration of port 465 for implicitly encrypted mail submission.